* Θρίαμβος (Thríambos): This is probably the closest single word. It refers specifically to a Roman triumphal procession, but the concept encompasses the idea of victory and celebration.
* Νίκη (Níki): This means "victory," and often implies the triumph *over* something or someone.
* Words related to victory or conquest might also be used depending on the context, such as ἐπιτυχία (epitychía) meaning success or κατάκτηση (katáktēsi) meaning conquest.
Therefore, θρίαμβος (Thríambos) is the best single word translation, but the best choice will always depend on the specific nuance of "triumph" you wish to convey.